You can get RO water at many grocery stores, Wal-Marts, and the like. If you see a big water dispenser where they sell water by the jug, that's usually it.
RO water is an excellent way to go if your tap water has a lot of chemicals or you have had problems with phosphates and/or silicates in your tank.
Another approach is simply to use a chloromine removing water conditioner, mix in the salt, and let the water sit for seven days (ideally with a heat source and circulation). This leaves all the trace elements in the water, but might allow less severe chemical treatments done by your public water supply to break down. This is less 'safe' than RO, but is better than pouring straight from the tap.
RO/DI filters cost about $150 and you can find them at many hardware store or buy them online.